Automatic frequency control circuits



Dec'. 23, 1941.

D.V E. FOSTER AUTOMATIC FREQUENCY CONTROL CIRCUITS f Filed March-15, 1957 2 sheets-,sheet 1 wrm l QERQ N H .u .N Fi

INVENTOR DuDLEY E. FOSTER BY #QZ ATTORNEY Dec. 23,` 1941. D. E. FOSTER AUTOMATIC FREQUENCY CONTROL CIRCUIS Filed March 1s; 1937 2 sheetsI-'sheet 2 .TO I. F. SOURCE AFC AAAAAAAA R E H R ow Mm T N EL/ ma ,n lm .A U D. Y B

Patented Dec. 23, 1941 AUTOMATIC FREQUENCI.' (ION'IRDL1 CIRCUITS. v

Dudley E. Foster, Morristown, N. J., assignerA to Radio Corporation of America, a corporation of Delaware ApplicationrMarch 13, 1931",J Serial No. 130,630

(Cl. Z50- 40) 7 Claims.

My present invention relates to automatic frequency control` arrangements in general, and

more particularly to control circuits adapted accurately to tune superheterodyne receivers at selected station adjustments.

In the past there have been disclosed various types of circuits whose function it was accurately to tune a receiver of the superheterodyne type when the tuningl device of. the latter was. adjusted to a desired` station setting. In, general such prior circuits employ a discriminator whose function it is to derive a` direct. current voltage from I. F. (intermediate frequency) energy; the voltage varying in polarity and value with thev sense and` amount of the frequency departure of the I.4 F. energy from a predeterr'ninedV I. F. value. The direct currentvoltage output of the discriminator is used to vary the gain of a frequency control tube in a manner such that the latter acts, to adjust the oscillator tank circuit frequency. The tank circuit frequency is adjusted so as. to compensate for the I. F. shift thereby accurately tuning the` receiver as soon as the tuning means is adjusted approximately to the desired station setting.

Y One of the main objects of my present inven.- tion is to provide an automatic frequency control circuit with -a discriminator deriving direct current voltages from I. F. energy whose frequency shifts from an assigned value; the direct current voltages beingV employed to` vary the operating biases of at least two control electrodes of a frequency control tubewhose output circuit and two control electrodes are so electrically related to the oscillator tank circuit as to produce positive andnegative reactive effects thereacross.

.Another important object of my invention isI to provide an improved frequency controltubecircuit for the local oscillator of a superheterod-yne receiver, wherein alternatingrvoltage developed across an impedance path in the oscillator tank circuit is impressed on the control electrodes of a pair of electron discharge devices, the output currents of said devices being employed to `produce reactance effects of predetermined sign across the tank circuit, and means being utilized to control the gain of said devices thereby to regulate the frequency of said tank circuit.l

Another object of the invention is to provide in combination with a tuned circuit having an impedance path across which alternating voltage is developed by current flow in the circuit, a pair of electron discharge devices whose electrodes may be disposed in a common tube envelope, the voltage being applied in push-pull manner to the control electrodes of said` devices` if the. output currents oi the latter are to be. impressed inl parallel on` said tuned circuit, or the alternating voltage. being applied in parallel to said control electrodes if the said outputcurrents are to be impressed in push--pull fashion on` said tuned circuit.

Still other objects' are generally to, improve `the eiliciency and operation of automatic frequency control circuits for superheterodynereceivers, and more especially to provide` frequency control circuits capable of reliable functioning and economical in construction.

The= novel features which` I believe to be characteristic of lmy invention are set forthl in Darticularity in the appended claims the invention itself, however, as to both` its organization and methodv of operation will best be understood by reference to they following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may becarried into effect.

In the drawingsv Fig. 1 shows a superheterodyne receiver embodying one form of the invention,(

Fig. 2 illustrates amodication, y

Fig. 3 shows the circuit arrangement` of Fig. l applied to a. different type` of frequency control tube.

Referring now to the. accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, Fig. 1 shows4 in conventional form a superheterodyne receiverpto which the invention is applied. In general, the receiver may comprise the usual signal collector followed by one or more stages of tunable radio frequency'amplication, and the receiver tuning range may cover 550 tol500 k. c. (the broadcast band) or the receiver may be of the multi-band type. The amplified signals are fed to a i'lrstl detector, orl mixer, I provided with a tunable input circuit` 2. The tuning condenser 3 is included in the circuit; its: rotors are mechanically coupled, in` the well known manner, to the same tuning shaft vcarrying the radio amplifier and local oscillator tuning condenser rotors. Local oscillations are impressed on` the mixer I by a local oscillator 4; the latter is provided with a tunable tank circuity 5, The coil. 'l and condenser B comprise the. latter, and it` will be understood that. the oscillator may be of any well known construction.- The 'low alternating poten-v tial side ofthe tank circuit 5 is shown grounded,

and it will be understood that` the tank circuity is connected between the grid and cathode of the oscillator tube as in the usual oscillator circuit. The numeral 8 denotes the local oscillation injection path to the first detector.

The numeral 9 represents the uni-control device for simultaneously adjusting the positions of the rotors of condensers 3 and 6 lto tune the signal and oscillator circuits through the receiver frequency range. Those skilled in the art are fully aware of the need for maintaining the frequency ranges of the signal circuits (as circuit 2) and oscillator tank circuit respectively different by a desired I. F. value. The usual padder condensers would be employedy in the oscillator tank circuit 5 to maintain the I. F. constant in value at all settings of device 9. The I. F. energy may be given a frequency value chosen from a range of '75 to 465 k. c. The I. F. energy output of first detector is transmittedthrough one, or more, I. Rampliers I to the second detector. The `lattercircuitis not shown since those skilled in the art fully appreciateitsv construction and that of the following networks;v the second detector may be followed by one or more audio amplifiers and terminated by a reproducer of any desired type. TheI. F. output circuit is notonly coupled by condenser I2 to the second detector, but is coupled to the discriminator which derives the direct current voltages, of requisite polarity and value from the I. F. energy.

The frequency control tube 2| 'is controlled in its effect on the frequency of the tank circuit 5 by the AFC (automatic frequency control) voltage produced by the discriminator. In general as the tuning device 9 is adjusted to a desired station setting, the effect of -the AFC circuit is to pull the oscillator tank frequency towards that 'frequency which will produce I. F. energy ofthe operating value. 'I'he same action happens if the local oscillator frequency shifts at a desired settingof the tuning device 9. The tuning mecha- 'nism of the receiver may, if desired, bel of the type shown in my application Serial No. 73,998, filed December 23, 1935.

Considering, now, the details of the AFC networks, the discriminator comprises a` pair of diodes I3 and |4`;` these diodes, if desired, may be embodied in a 6I-I6 type tube; There is connected between the anode and grounded cathode of diode I3 av series path including tuned circuit I5 and resistor I8; the bypass condenser I1 being connected across the resistor I8. Between the anode and grounded cathode o f diode |4 is connected the tuned circuit I6 and resistor 20 arranged in series; bypass condenser I9 being connected across the resistor 20;' Each of the tuned circuits I5 and I6 isi-magnetically coupled to the I. F. tuned circuit II. However, one of the diodeA input circuits is resonated to a frequency above the assigned I. F. value, whereas the other diode input circuit is resonated to a frequency below the I. F. Value'. In this way'a shift in I. F. value from-the assignedfrequency results in production `of direct current voltage across that diode resistor which isconnected to the diode input circuitresonated in the direction of shift. The voltages 'across resistors I8 and 20 are negative with respect to ground.

. The frequency controltube 2|. is of the 6A6 type; Vit is a double triode tube provided with a common cathode. The cathode is grounded through coil 22; platevoltage is'applied to the platesof the tube through choke v36. Both plates are connected by a common lead 23 through the isolating condenser .35 'to the high alternating potential side of the oscillator tank circuit 5. The grids 2'4 and 25, of the two triodes, are not only subject to the direct current voltages of the discriminator diodes, but have, in addition, impressed thereon alternating current voltages developed across the reactive path in the tank circuit 5. The said reactive path includes resistor 26, condenser 21 and condenser 2'8 arranged in series between the high alternating voltage side of tank circuit 5 and ground. The grid 24 is connected by lead 29 to the junction of resistor 26 yand condenser 21; the grid 25 is connected to the grounded side of condenser 28 through condenser 30. The grids 24 and 25 have applied to them positive and negative alternating current potentials .produced by the ow of oscillator currentthrough the path 2'6-21-28.

'I'he grid 24 is connected by lead 3| to the anode side of diode resistor 20; the resistor 32 included in the AFC lead 3| functions to suppress all pulsating components in the bias voltage applied-through lead 3|. The AFC lead 33, which includes filter resistor 34, connects grid 25 to the anode side of diode resistor I8. In this way the discriminator functions to control the current flow from tube 2| to the oscillator tank circuit 5. The tube 2|, in so far as alternating current is concerned, has a push-pull input connection to the path 26-21-28 across the tank circuit 5, whereas the output electrodes of the tube 2| are connected in parallel relation to the tank circuit.

From a generic viewpoint the action of the control tube 2`| is to produce, or simulate, across the tank circuit 5 positive and negative inductive reactances. The presence or absence of a simulated reactance of predetermined sign depends upon the AFC bias. In other words, the magnitude and sign of the reactance produced across the oscillator tank circuit 5 not only depends upon the alternating voltage applied to one of the grids of tube 2|, but it also depends upon the magnitude of the AFC bias. When the output frequency of first detector I is that to which circuit is tuned, the voltages across resistors I8 and 20 are equal by virtue ofthe fact that circuits I5 and I6 are symmetrically spaced in resonant frequency on each side ofthe normal frequency, and, therefore, the net effect of tube 2| on the oscillation frequency of circuit 1 6 is nil. Circuit I6 should be tuned to a frequency higher than the normal I. F. by approximately 4 k. c., and circuit I5 lower than normal I. F. by the same amount. If the receiver, then, be tuned so that the oscillation frequency of circuit 5 is too high, the resultant I. F. will be too high, and there will be more energy in circuit I6 than in circuit I5. The negative bias from diode I4 applied to grid 24 will then be higher than that applied to grid 25 from diode I3. The eiect of making grid 24 more negative and grid'25 less negative is to increase the effective inductance due to tube 2| across inductance 1, thus decreasing the oscillation frequency until the resultant I. F. is normal, which means the energy in circuits I5 and 'I6 is the same. It will be noted that change in signal amplitude changes voltages derived from diodes I3 and I4 together, and since the effect of each side of tube 2| is to vary the frequency in the opposite direction from that of the other side there is no change in oscillation frequency with signal amplitude. If the frequency of oscillator!! is too low, the effective inductance across circuit 5 is decreased.

It` is not essential to the present invention th'at the frequency control tube having its grids in push-pull relation to the tank circuit'. In Figi. 2 there is sliown' a modification wherein rthe grids' 24 and 25 of tube 22| arelconnected in par-l `of a common lead 42, to the junction of resistor 26vl and condenser 21'. The plates 40 and4| are connected 'to the opposite terminals of the primary winding 1' of a transformer whose sec-v ondary winding is the'- coil 1 ofthe tankl circuit'. Themidpoint ofcol 1' is connected to a source of positive potential.

The common cathode of tube 2| is connected to ground through the grid bias network 22.- Th'e AFC lead 3| is connected'lto grid 24, while the AFC lead 33 is connected to grid 25. In this modication of the inventionboth grids 24 and 25 are at the same alternating current voltage polarity by virtue of their parallel connection to the junction of resistor 26' and condenser 21. However, due to the push-pull coupling of plates 4U and 4| to the tank circuit 5, the inductive reactancessimulated across the tank circuit will be of different sign. The discriminator network of Fig. 2 differs from that of Fig. l in that side tuned circuits are not used. The discriminator is similar to that sh'own in Seeley Patent No. 2,121,103, June 21, 1938, except that the junction of resistors |8 and 2U is at ground potential instead of one of the cathodes. When the frequency of the resultant I. F. differs from that to which circuits l5 and are resonated, the positive potential across one of the resistors I8' and 20 is higher than that across th'e other, producing an unbalance in the grids of tube 2| and a change in frequency in the direction to bring the I. F. back to normal. The polarity of coil I5" relative to the coil of circuitl determines which resistor, I8 or 20', will have to be most positive when the frequency is higher than "normalv I. F. Similarly the polarity of coil 1 Vdetermines whether the oscillation frequency will increase or decrease when gridy 24 is more positive than grid 25. y V

The grid is connected by AFC lead 33 to the cathode side of resistor |8; while the grid 24 is connected by AFC lead 3| to the cathode side of resistor 20. In the type of discriminatoishown in Fig. 2, the coupled circuits and l5 are tuned to the same I. F.; the midpoint of coil I5" is connected to the high alternating potential side of circuit by condenser I6'. The midpoint of resistors |8'20 is grounded, and the ground point is connected to the midpoint of coil l5". Condenser |1 shunts resistors |820'. It is not believed necessary to describe the action of the discriminator other than to point out that at resonance the direct currentvoltages across I8' and 20' are equal in value but opposite in polarity. When the applied I. F. energy shifts in frequency from the assigned frequency, then the ydrop across |8 or 20' will dominate depending on the direction of sh'ift. This is all clearly described in the said Seeley application, and need not be repeated here.

The arrangement shown in Fig. 3 is similar electrically to that of Fig. 1 for the reason that the input grids of the frequency control tube are connected in push-pull relation to the oscilcuit coil 1.

lator `tank circuit, whereas` the output electrode circuit isconnected` in parallel relation te the tank circuit. However',v it differs'A from` thefar.- rangement shown in Fig. 14 in that the tube: 50. whichis employed may beefthe 6A7` type wherein asingle plate 5| hasdisposed between it and the cathodethe various gridelectrodes which are-, to-beused: for the control functions. The oscillator tube` 52 has the tank circuit 5 connected between its` grid and, cathode. To plateV 5|E of the frequency control tube 50 is` applied the desired positive potential through the tank cir-` The condenser 53 functions as a padder condenser, and the condenser 54 acts asA a direct current blocking condenser. The remaining elements of the. oscillator circuit are well known, and need no further description. The series path to ground across the` tank circuit 5 includes the resistor 26, condenser 21 and condenser 28. The junction. of resistor 26 and condenser 21 is connected by lead 55 and condenserl56 to the control gridA 51 of the frequency control tube 50.

The ground side of condenser 28 is connected by lead 58 and condenser 59 to the control grid 6|) of the control tube. The cathod of the tube is connected to ground through the radio frequency choke coil 6|. 'I'he grids 51 and 60 are separated by a positive grid, and the suppressor grid and control grid 51 are also separated by a positive grid. The control grid 51 is connected by lead 3| to the source of negative AFC' bias, as by connecting the lead 3| to the anode side of the diode resistor 20 of Fig. 1. The control grid 60' is connected, as by lead 33, to the source of negative AFC bias in the manner shown in Fig. 1.

It will, therefore, be seen that the path 26- 21-28` in Fig. 3 provides a means for producing out-of-phase voltage on the grids 51 and 60 of control tube 50. The AFC bias of each ofthese control grids is so proportioned that in the normall tuning condition, that is when waves of the assigned I. F. are received, the product of the effect of both grids on the mutual conductance of tube 50 and the impedance across condenser 21 or 28 respectively, are equal. In other words if Smi is the mutual conductance of tube 50 due to grid 60, and Smz is the conductance due to the other grid 51, then Sml This relation exists when no differential control bias is applied. If the control characteristics of grids 6|] and 51 are different, as they will be in a tube of the 6A7 type, the AFC biases should be similarly proportioned to the control characteristics of grids 51 and 60 by adjusting the value of the diode resistors I8 and 20. The explanation given in connection with Fig. 1 as to the mode of operation of the control circuit is applicable to the modification in Fig. 3.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In .combination with a resonant circuit tuned to a desired frequency, an electron discharge tube provided with a cathode, at least two control grids andan output electrode, fanoutput circuit connected to the output electrode, said resonant circuit being coupledto Athe output cirl to a desired frequency, an electron discharge tube provided with a cathode, a plate and at least two control grids arranged in the electron stream to the plate, an output circuit, said resonant circuit being coupled to the output circuit, means connected directly across said resonant circuit for developing alternating current voltage from alternating current owing in said resonant circuit, means for applying said voltage to said two control grids in out-'of-phase relation, additional means for varying the voltages of said grids in opposite polarity sense, and said `plate being connected to a point in said output circuit.

3. In combination with a resonant circuit tuned to a desired frequency, an electron discharge tube provided with a cathode, at least two control grids and one output electrode, an output circuit, said resonant circuit being coupled to the output circuit, means connectedto said resonant circuit for developing alternating current voltage 4from alternating current owing in said resonant circuit, means for applying said voltage to said two control grids in out-of-phase relation, additional means for varying the biases of said grids in opposite polarity sense, said one output electrode being connected to said output circuit, and said control grids being disposed in the electron stream flowing between the tube cathode and said output electrode.

4. In combination with an oscillator provided with a resonant tank circuit, a frequency control tube provided with at least a cathode, a pair of control grids and at least one output electrode, means coupling said output electrode to said tank circuit, a reactive path connected directly in shunt to said tank circuit and developing alternating current voltage from current flowing in said tank circuit, means for applying said alternating current voltage to said control grids, and additional means for varying the voltages of said grids in opposite polarity senses.

anew-ius i5. .Invcombi'n'ation with an oscillator provided with aresonant tank circuit, a frequency control tube provided with at least a cathode, a pair of control grids and at least one output electrode, means coupling said output electrode to said tank circuit, a reactive path connected directly in shunt to said tank circuit and developing alternating currentvoltage from current flowing in said tank circuit, means for applying said alternating current voltage to said control grids, additional means for varying the direct current potential of said grids in opposite polarity senses, said reactive path including at least two reactances of the same sign, and the connections between said control grids and said reactances being such that the alternating voltage impressed on the two grids is in phase opposition relation.

6. In combination with an oscillator provided with a resonant tank circuit, a frequency control tube provided with at least a cathode, a pair of control grids and at least one output electrode, means coupling said output electrode to said tank circuit, a reactive path connected to said tank circuit and developing alternating current voltage from current owing in said tank circuit, means for applying said alternating current voltage to said control grids, additional means for varying the direct current potential of said grids in opposite polarity senses, said reactive path comprising a pair of condensers arranged in series across the tank circuit, and said control grids being connected to points of said condensers which are of opposite phase.

7. In combination with an oscillator provided with a resonant tank circuit, a frequency control tube provided with at least a cathode, a pair of control grids and at least one output electrode, means coupling said output electrode to said tank circuit, a reactive path connected to said tank circuit and developing alternating current voltage from current flowing in said tank circuit, means for applying said alternating current voltage to said control grids, additional means for varying the direct current potential of said grids in opposite polarity senses, said tube including a positive shielding electrode between the said control grids, the cathode 0f the tube being connected to a point of xed potential through a radio frequency choke coil, and said reactive path comprising a pair of condensers in series relation across the tank circuit.

DUDLEY E. FOSTER. 

